This invention in general relates to optical communications and in particular to bulk optic echelon multiplexers.
Wavelength division multiplexing (WDM), the simultaneous transmission of several signals on a single path, is a technology that is fundamentally important in fiber based communications systems because of its impact on system configuration, performance, and cost. One major advantage of this technology is its ability to increase system capacity by increasing the number of channels that can be carried per fiber. With increased capacity, fiber requirements and associated fiber costs decrease, and already installed systems can more easily be upgraded to handle subsequent increases in traffic. Additionally, different modulation schemes can be used on assignable channels to enhance flexibility and overall system design.
Wavelength selective devices for multi/demultiplexing may be classified in a variety of categories according to physical properties and function and include such things as multiwavelength sources or detectors, dielectric filters, and angularly dispersive devices like prisms and gratings. As evidenced by the literature, high resolution line and Fresnel gratings have been proposed and used. However, their fabrication requires submicron precision, since the line periodicity required for adequate resolution is of the order of a wavelength, and the accuracy for good quality, low noise gratings must be considerably better than one wavelength of light. Consequently, it is relatively difficult to exploit conventional photolithographic integrated optics fabrication technology to manufacture line gratings to the precision required for WDM applications.
Gratings with periodicities larger than the normal line grating are also known and have been proposed for use as optical spectrometers and fiber optic multiplexers. These include the echelon, echelle, and echelette. Typically, their use for such applications has been that of replacing simple low order diffraction gratings.
Echelon gratings were first proposed by Michelson as a means for creating a blazed grating with very high wavelength dispersion. One should note that the dispersion of an echelon grating is no different than that of a normal grating used at the same angle of light incidence. Normal gratings, however, exhibit a low diffraction efficiency when used at a high order of diffraction. By contrast, an echelon grating can diffract theoretically up to 100% of incident light into a single diffraction order.
The ability to manufacture reasonable quality echelon gratings has not existed until the recent introduction of ruling equipment that uses coherent light in a feedback loop to control the position of the ruling machine to an accuracy much more precise than one wavelength of light. The actual machining is accomplished using a small, precisely formed diamond tool. Typically, the positional accuracy allows the light level of ghosts to be 1,000 to 10,000 times lower than that of the principal blazed diffraction beam. However, the lack of exactly flat tools for machining promotes the diffraction of light into orders other than the desired blazed order, and therefore typically reduces the efficiency for blazed diffraction to less than 50%.
Known commercially available echelon gratings are blazed to operate at an angle of 63.degree. or more from normal incidence as a means for increasing the wavelength dispersion. In many cases, operation at such high angles of incidence implies that the width of a facet (perpendicular to the grating lines) of the grating is not large compared to a wavelength of light. Moreover, with machined gratings, each facet is likely to be irregularly non-flat with the result that the effective width of the flat may be even smaller than if calculated using the total step width. Under these circumstances, the electrical resistivity of metalized surfaces at optical frequencies, in the direction in which current flow is restricted, produces a decreased reflectivity for light polarized in that direction. Conversely, since there is essentially no geometric current restriction along the lines of the grating, the reflectivity for light polarized in that direction is unchanged with respect to a flat mirror. The net result is that echelon gratings are often manufactured so as to exhibit an inherent sensivitity to the state of polarization of an incident light beam.
The fiber optics telecommunications market is focused on two specific optical bands centered on 1.30 and 1.55 microns because these bands correspond to the lowest loss wavelength range for fused silica fiber. Because there is a large amount of fiber installed in the ground which operates at 1.30 microns, the wavelength of lowest dispersion in typical single mode fiber, and because the lowest fiber loss is not at 1.55 microns, there is a great deal of uncertainty as to which band will become preeminent for long haul and short haul telecommunication transmission. Moreover, there is a concensus of opinion that the fiber installed for use at 1.30 microns will be useable at 1.55 microns if narrow line laser sources (distributed feedbak lasers) are used as transmitters since then the higher dispersion at 1.55 microns would no longer limit the bandwidth-length transmission distance. Fiber optic systems designers consequently would like to be able to hedge on system design by using components that will operate at either 1.30 or 1.55 microns or at both wavelengths simultaneously.
In view of the foregoing, it is a primary object of the present invention to provide a bulk optic echelon grating suitable for use in optical communications systems for multi/demultiplexing purposes.
It is another object of the present invention to provide an easily fabricated multi/demultiplexing device for use in optical communications.
Yet another object of this invention is to provide an echelon multi/demultiplexer that simultaneously operates at a number of different wavelengths at the same position in the focal plane.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter when the detailed description to follow has been read.